Current design methodologies for microwave circuit design of MMIC amplifiers generally include the development of a matching network between the complex impedance of the active devices or transistors and a nominal 50 ohm connection impedance. Likewise, antennas are designed to provide a match between the 377 ohm impedance of free space and 50 ohms.
Often these antennas requite a transformation between balanced and unbalanced lines or between coaxial and micro-strip transmission lines. Also, note that an antenna element and an amplifier may have a long length transmission line connecting them together. The transmission line and any intermediate elements tend to degrade system performance, with degradation for very broad bandwidth applications being even greater.
In short, connections between an antenna aperture and receiver or transmitter electronics often includes multiple interfaces, components and connection lines, all of which limit and degrade system performance with their respective losses and parasitic effects. If these effects could be eliminated, the result would be a significantly improved system noise figure, out power, linearity, flatness, match, and bandwidth.
More particularly, electronics utilized in connection with an antenna, either transmitter or receiver, were previously located remotely from the antenna itself. Thus, between the electronics and the antenna there was typically a long piece of coaxial cable, connectors attaching the cable to the electronic boxes, and baluns or other devices used to match the nominal 50 ohm coax impedance to the impedance of the antenna at the antenna feed points.
While it is possible to feed an antenna with an unbalanced line, all of the separation between the antenna itself and the associated interfacing electronics adds insertion loss, resulting in lost sensitivity for a receiver and lost transmit power for a transmitter, noting that all the interfaces add up to decrease the bandwidth of operation. If the bandwidth of operation is decreased, this also decreases the ability to match the antenna to associated electronic devices.
As described in a patent application entitled “Integrated Electronics Matching Circuit At Antenna Feed Point For Maintaining Wide Bandwidth And A Low VSWR Operation”, invented by David F. Meharry, Edward Urbanik, and Arturs Dinbergs, Ser. No. 12/468,109 filed May 19, 2009 and assigned to the assignee hereof, the contents of which are incorporated herein by reference, to provide a wide bandwidth match, integrated electronics matching circuits are placed at the feed point of the antenna above the ground plane normally utilized in printed circuit microwave antennas. The purpose of the location of the integrated electronics above the integrated circuit ground plane is to reduce to zero the connection length from the feed point to the place where the matching is done. It is noted in this patent application that the conventional length from where the initial or complete matching is done beneath the ground plane up to the antenna feed point could be considerable and sometimes exceeded 100 mils.
While this application was directed primarily to an integrated electronics matching circuit, there is still a necessity to place an amplifier having a balanced output at the feed of the antenna, noting that most antennas are designed to have a balanced input at their feed point.
There is therefore a necessity to provide electronic circuits the feed point of the antenna which present a balanced interconnection topology. This includes electronics functioning as an amplifier, rather than functioning as a balun or tuning circuit.
Assuming that one can get the electronics close to the feed point of the antenna, another benefit is that if these electronics involve balanced electronics, one can select a more appropriate impedance level for the antenna. If one provides balanced electronics at the feed point one can obtain a wide bandwidth when going to higher impedances. Thus, while nominally working at a 50 ohm input impedance, providing a balanced interconnect circuit one can provide a 100 ohm output impedance which is useful in improving bandwidth characteristics for the antenna.
There are two common broadband microwave antennas for which the above set of problems is severe. The first is the Vivaldi notch antenna which is a tapered notch. The second is a bi-cone antenna. If a way could be found to provide a balanced amplifier circuit directly coupled to the feed points of these antennas, then it would be possible to eliminate interfaces which limit and degrade system performance.